Two designs of heat exchanger are presently in general use for reboiler-condensers in air separation and similar cryogenic, and chemical applications. The most common of these is the plate-fin brazed aluminum heat exchanger fabricated by disposing corrugated aluminum sheets between parting sheets to form a plurality of fluid passages.
The second type of heat exchanger in current use is a vertical shell and tube reboiler. To achieve a sufficiently low temperature difference with this design, enhanced surfaces are used. A porous boiling surface is applied to the inside of the tubes, and longitudinal flutes are used on the outside of the tubes. The disadvantages of the shell and tube design are the limited heat transfer surface which can be accommodated in a distillation column and the high cost of construction of the heat exchanger. In addition, this type of exchanger is subject to accumulation of thick liquid condensate films in the lower regions of the exchanger.
A third type of exchanger which is believed to have seen some application for reboiler-condensers in cryogenic separation plants and which is available commercially in the "BAVEX" type exchanger. This configuration is described in U.S. Pat. No. 3,720,071. Specially corrugated sheets are juxtaposed to define passages for the boiling oxygen and the condensing nitrogen. This exchanger is apparently also subject to the build-up of thick condensate films, since various attempts are described to put ribs, projections, and the like, on the condensing side of the corrugated sheets to remove the condensate from the sheets. The exchanger is intended to operate with boiling in the conventional manner from the plain metal surface of the corrugated sheets.
Russian Pat. No. 1,035,398 describes a plate type reboiler-condenser. The condensing passages have perforated corrugated inserts and inclined channels machined into the plates which are intended to drain condensate to the sides of the exchanger. The boiling passages have ribbed projections on the plates, additionally covered with a porous enhanced boiling surface.
U.S. Pat. No. 4,371,034 describes a plate type evaporator with an enhanced porous surface applied to the boiling side. The boiling liquid is recirculated in thermosyphon fashion. Since the heating medium can be a condensing stream, the proposed heat exchanger can be used as a reboiler-condenser. The heat exchanger is a combination of an enhanced boiling surface on the plates of a conventional exchanger of the plate type. The gasketed construction is unsuitable for cryogenic service. No enhancement is proposed for the hot, i.e. condensing, side of the exchanger.
West German Pat. No. 3,011,011 describes a plate type reboiler-condenser for air separation service, where individually extruded plates are stacked and brazed together to form vertical boiling and condensing channels with small rectangular cross sections. Voids in the extruded plates comprise the condensing channels, and longitudinal thick ribs on the plates comprise fins in the boiling passage. These fins are much thicker than those used in conventional plate-fin brazed aluminum exchangers. The boiling channels defined between the ribs of the extrusions do not communicate with one another and could pose a safety problem if even one of the small channels were to be inadvertently closed off and permit dry boiling to occur. Putting an enhanced boiling surface on the ribbed side of the plates is disclosed; however, no enhancement is provided on the condensing side.